Cable with recyclable covering

ABSTRACT

The invention describes a cable with recyclable covering, particularly for transporting or distributing medium or high voltage energy, in which at least one covering layer is based on thermoplastic polymer material comprising a propylene homopolymer or a copolymer of propylene with ethylene or an α-olefin other than propylene in mixture with a dielectric liquid. The cable of the invention possesses superior mechanical and electrical properties, including high dielectric strength, in particular enabling it to be used at high operating temperature.

[0001] The present invention relates to a cable with recyclablecovering. In particular, the invention relates to a cable fortransporting or distributing medium or high voltage electric energy,wherein an extruded covering layer based on a thermoplastic polymermaterial in admixture with a dielectric liquid with superior mechanicaland electrical properties is present, enabling, in particular, the useof high operating temperatures and the transportation of high powerenergy.

[0002] The requirement for products of high environmental compatibility,composed of materials which, in addition to not being harmful to theenvironment during production or utilization, can be easily recycled atthe end of their life, is now fully accepted in the field of electricaland telecommunications cables.

[0003] However the use of materials compatible with the environment isconditioned by the need to limit costs while, for the more common uses,providing a performance equal to or better than that of conventionalmaterials.

[0004] In the case of cables for transporting medium and high voltageenergy, the various coverings surrounding the conductor commonly consistof polyolefin-based crosslinked polymer, in particular crosslinkedpolyethylene (XLPE), or elastomeric ethylene/propylene (EPR) orethylene/propylene/diene (EPDM) copolymers, also crosslinked. Thecrosslinking, effected after the step of extrusion of the polymericmaterial onto the conductor, gives the material satisfactory performanceeven under hot conditions during continuous use and with currentoverload.

[0005] It is well known however that crosslinked materials cannot berecycled, so that manufacturing wastes and the covering material ofcables which have reached the end of their life can be disposed of onlyby incineration.

[0006] Electric cables are also known having their insulation consistingof a multi-layer wrapping of a paper or paper/polypropylene laminateimpregnated with a large quantity of a dielectric liquid (commonly knownas mass impregnated cables or also oil-filled cables). By completelyfilling the spaces present in the multi-layer wrapping, the dielectricliquid prevents partial discharges arising with consequent perforationof the electrical insulation. As dielectric liquids products arecommonly used such as mineral oils, polybutenes, alkylbenzenes and thelike (see for example U.S. Pat. Nos. 4,543,207, 4,621,302, EP-A-0987718,WO 98/32137).

[0007] It is however well known that mass impregnated cables havenumerous drawbacks compared with extruded insulation cables, so thattheir use is currently restricted to specific fields of application, inparticular to the construction of high and very high voltage directcurrent transmission lines, both for terrestrial and in particular forunderwater installations. In this respect, the production of massimpregnated cables is particularly complex and costly, both for the highcost of the laminates and for the difficulties encountered during thesteps of wrapping the laminate and then of impregnating it with thedielectric liquid. In particular, the dielectric liquid used must havelow viscosity under cold conditions to allow rapid and uniformimpregnation, while at the same time it must have a low tendency tomigrate during installation and operation of the cable to prevent liquidloss from the cable ends or following breakage. In addition, massimpregnated cables cannot be recycled and their use is limited to anoperating temperature of less than 90° C.

[0008] Within non-crosslinked polymeric materials, it is known to usehigh density polyethylene (HDPE) for covering high voltage cables. HDPEhas however the drawback of a lower temperature resistance than XLPE,both to current overload and during operation.

[0009] Thermoplastic low density polyethylene (LDPE) insulatingcoverings are also used in medium and high voltage cables: again in thiscase, these coverings are limited by too low an operating temperature(about 70° C.).

[0010] WO 99/13477 describes an insulating material consisting of athermoplastic polymer forming a continuous phase which incorporates aliquid or easily meltable dielectric forming a mobile interpenetratingphase within the solid polymer structure. The weight ratio ofthermoplastic polymer to dielectric is between 95:5 and 25:75. Theinsulating material can be produced by mixing the two components whilehot either batchwise or continuously (for example by means of anextruder). The resultant mixture is then granulated and used asinsulating material for producing a high voltage electric cable byextrusion onto a conductor. The material can be used either inthermoplastic or crosslinked form. As thermoplastic polymers areindicated: polyolefins, polyacetates, cellulose polymers, polyesters,polyketones, polyacrylates, polyamides and polyamines. The use ofpolymers of low crystallinity is particularly suggested. The dielectricis preferably a synthetic or mineral oil of low or high viscosity, inparticular a polyisobutene, naphthene, polyaromatic, α-olefin orsilicone oil.

[0011] The Applicant considers as still unsolved the technical problemof producing an electric cable with a covering made from a thermoplasticpolymer material having mechanical and electrical properties comparableto those of cables with an insulating covering of crosslinked material.In particular, the Applicant has considered the problem of producing acable with a non-crosslinked insulating covering having good flexibiltyand high mechanical strength under both hot and cold conditions, whileat the same time possessing high dielectric strength, without usingproducts potentially polluting during the life cycle of the cable, i.e.from its production to its disposal.

[0012] In view of said problem, the Applicant considers that theaddition of dielectric liquids to polymer materials as proposed in thecited WO 99/13477 gives totally unsatisfactory results. In this respect,the Applicant maintains that adding a dielectric liquid to an insulatingmaterial should both determine a significant increase in its electricalproperties (in particular its dielectric strength), without changing thematerial characteristics (thermomechanical properties, manageability)and without resulting in exudation of the dielectric liquid. Inparticular, the resultant cable should give substantially constantperformance with time and hence high reliability, even at high operatingtemperatures (at least 90° C. and beyond).

[0013] The Applicant has now found it possible to solve said technicalproblem by using, as recyclable polymer base material, a thermoplasticpropylene homopolymer or copolymer mixed with a dielectric liquid ashereinafter defined. The resultant composition possesses goodflexibility even when cold, excellent thermomechanical strength and highelectrical performance, such as to make it particularly suitable forforming at least one covering layer, and in particular an electricalinsulating layer, of a medium or high voltage cable of high operatingtemperature, of at least 90° C. and beyond. The dielectric liquidsuitable for implementing the invention has high compatibility with thebase polymer and high efficiency in the sense of improving electricalperformance, consequently allowing the use of small quantities ofadditive such as not to impair the thermomechanical characteristics ofthe insulating layer.

[0014] High compatibility between the dielectric liquid and the basepolymer ensures homogeneous dispersion of the liquid in the polymermatrix and improves cold behaviour of the polymer. Moreover, as thedielectric liquid suitable for forming the cable of the invention isfree of polar groups, it absorbs water in extremely small quantities,hence preventing formation of insulation defects due to the presence ofsteam which normally forms during the process of high temperatureextrusion.

[0015] According to a first aspect, the invention therefore relates to acable (1) comprising at least one electrical conductor (2) and at leastone extruded covering layer (3, 4, 5) based on a thermoplastic polymermaterial in admixture with a dielectric liquid, wherein:

[0016] said thermoplastic material comprises a propylene homopolymer ora copolymer of propylene with at least an olefin comonomer selected fromethylene and an α-olefin other than propylene, said homopolymer orcopolymer having a melting point greater than or equal to 140° C. and amelting enthalpy of from 30 to 100 J/g;

[0017] said liquid comprises at least one alkylaryl hydrocarbon havingat least two non-condensed aromatic rings and a ratio of number of arylcarbon atoms to total number of carbon atoms greater than or equal to0.6, and preferably greater than or equal to 0.7.

[0018] According to a first embodiment, said extruded covering layerbased on said thermoplastic polymer material in admixture with saiddielectric liquid is an electrically insulating layer.

[0019] According to a further embodiment, said extruded covering layerbased on said thermoplastic polymer material in admixture with saiddielectic liquid is a semiconductive layer.

[0020] Preferably, the propylene homopolymer or copolymer has a meltingpoint of from 145 to 170° C.

[0021] Preferably, the propylene homopolymer or copolymer has a meltingenthalpy of from 30 to 85 J/g.

[0022] Preferably, the propylene homopolymer or copolymer has a flexuralmodulus, measured in accordance with ASTM D790, at room temperature, offrom 30 to 1400 MPa, and more preferably from 60 to 1000 MPa.

[0023] Preferably, the propylene homopolymer or copolymer has a meltflow index (MFI), measured at 230° C. with a load of 21.6 N inaccordance with ASTM D1238/L, of from 0.05 to 10.0 dg/min, morepreferably from 0.5 to 5.0 dg/min.

[0024] If a copolymer of propylene with an olefin comonomer is used,this latter is preferably present in a quantity of less than or equal to15 mol %, and more preferably of less than or equal to 10 mol %. Theolefin comonomer is, in particular, ethylene or an α-olefin of formulaCH₂═CH—R, where R is a linear or branched C₂—C₁₀ alkyl, selected forexample from: 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene,1-octene, 1-decene, 1-dodecene and the like, or combinations thereof.Propylene/ethylene copolymers are particularly preferred.

[0025] Preferably, said thermoplastic material is selected from:

[0026] a) a propylene homopolymer or a copolymer of propylene with atleast one olefin comonomer selected from ethylene and an α-olefin otherthan propylene, having a flexural modulus generally of from 30 to 900MPa, and preferably of from 50 to 400 MPa;

[0027] b) a heterophase copolymer comprising a thermoplastic phase basedon propylene and an elastomeric phase based on ethylene copolymerizedwith an α-olefin, preferably with propylene, in which the elastomericphase is present in a quantity of at least 45 wt % on the total weightof the heterophase copolymer.

[0028] The homopolymers or copolymers of class a) show a single-phasemicroscopic structure, i.e. substantially devoid of heterogeneous phasesdispersed as molecular domains of size greater than one micron. Thesematerials do not show in fact the optical phenomena typical ofheterophase polymer materials, and in particular are characterised bybetter transparency and reduced whitening due to local mechanicalstresses (commonly known as “stress whitening”).

[0029] Particularly preferred of said class a) is a propylenehomopolymer or a copolymer of propylene with at least one olefincomonomer selected from ethylene and an α-olefin other than propylene,said homopolymer or copolymer having:

[0030] a melting point of from 140 to 165° C.;

[0031] a melting enthalpy of from 30 to 80 J/g;

[0032] a fraction soluble in boiling diethyl ether in an amount of lessthan or equal to 12 wt %, preferably from 1 to 10 wt %, having a meltingenthalpy of less than or equal to 4 J/g, preferably less than or equalto 2 J/g;

[0033] a fraction soluble in boiling n-heptane in an amount of from 15to 60 wt %, preferably from 20 to 50 wt %, having a melting enthalpy offrom 10 to 40 J/g, preferably from 15 to 30 J/g; and

[0034] a fraction insoluble in boiling n-heptane in an amount of from 40to 85 wt %, preferably from 50 to 80 wt %, having a melting enthalpy ofgreater than or equal to 45 J/g, preferably from 50 to 95 J/g.

[0035] Further details of these materials and their use in coveringcables are given in European patent application 99122840 filed on Nov.17, 1999 in the name of the Applicant, incorporated herein forreference.

[0036] The heterophase copolymers of class b) are thermoplasticelastomers obtained by sequential copolymerization of: i) propylene,possibly containing minor quantities of at least one olefin comonomerselected from ethylene and an α-olefin other than propylene; and thenof: ii) a mixture of ethylene with an α-olefin, in particular propylene,and possibly with minor portions of a diene. This class of product isalso commonly known by the term “thermoplastic reactor elastomers”.

[0037] Particularly preferred of the said class b) is a heterophasecopolymer in which the elastomeric phase consists of an elastomericcopolymer of ethylene and propylene comprising from 15 to 50 wt % ofethylene and from 50 to 85 wt % of propylene on the weight of theelastomeric phase. Further details of these materials and their use incovering cables are given in European patent application 98830800 filedon Dec. 30, 1998 in the name of the Applicant, incorporated herein forreference.

[0038] Products of class a) are available commercially for example underthe trademark Rexflex^(R) of the Huntsman Polymer Corporation.

[0039] Products of class b) are available commercially for example underthe trademark Hifax^(R) of Montell.

[0040] Alternatively, as thermoplastic base material, a propylenehomopolymer or copolymer as hereinabove defined can be used inmechanical mixture with a low crystallinity polymer, generally with amelting enthalpy of less than 30 J/g, which mainly acts to increaseflexibility of the material. The quantity of low crystallinity polymeris generally less than 70 wt %, and preferably of from 20 to 60 wt %, onthe total weight of the thermoplastic material.

[0041] Preferably, the low crystallinity polymer is a copolymer ofethylene with a C₃—C₁₂ α-olefin, and possibly with a diene. The α-olefinis preferably selected from propylene, 1-hexene and 1-octene. If a dienecomonomer is present, this is generally C₄—C₂₀, and is preferablyselected from: conjugated or non-conjugated linear diolefins, such as1,3-butadiene, 1,4-hexadiene, 1,6-octadiene or their mixtures and thelike; monocyclic or polycyclic dienes, such as 1,4-cyclohexadiene,5-ethylidene-2-norbornene, 5-methylene-2-norbornene,5-vinyl-2-norbornene or their mixtures and the like.

[0042] Particularly preferred ethylene copolymers are:

[0043] (i) copolymers having the following monomer composition: 35-90mol % of ethylene; 10-65 mol % of an α-olefin, preferably propylene;0-10 mol % of a diene, preferably 1,4-hexadiene or5-ethylene-2-norbornene (EPR and EPDM rubbers are within this class);

[0044] (ii) copolymers having the following monomer composition: 75-97mol %, preferably 90-95 mol %, of ethylene; 3-25 mol %, preferably 5-10mol %, of an α-olefin; 0-5 mol %, preferably 0-2 mol %, of a diene (forexample ethylene/1-octene copolymers, such as the products Engage^(R) ofDow-DuPont Elastomers).

[0045] The alkylaryl hydrocarbon of the invention preferably has adielectric constant, at 25° C., of less than or equal to 3.5 andpreferably less than 3 (measured in accordance with IEC 247).

[0046] According to a further preferred aspect, the alkylarylhydrocarbon of the invention has a predetermined viscosity such as toprevent fast diffusion of the liquid within the insulating layer andhence its outward migration, while at the same time such as to enable itto be easily fed and mixed into the polymer. Generally, the dielectricliquid of the invention has a kinematic viscosity, at 20° C., of between1 and 500 mm²/s, preferably between 5 and 100 mm²/s (measured inaccordance with ISO 3104).

[0047] According to a further preferred aspect, the alkylarylhydrocarbon of the invention has a hydrogen absorption capacity greaterthan or equal to 5 mm³/min, preferably greater than or equal to 50mm³/min (measured in accordance with IEC 628-A).

[0048] According to a preferred aspect, an epoxy resin can be added tothe dielectric liquid suitable for forming the cable of the invention,generally in a quantity of less than or equal to 1 wt % on the weight ofthe liquid, this being considered to mainly act to reduce the ionmigration rate under an electrical field, and hence the dielectric lossof the insulating material.

[0049] In a preferred embodiment, the dielectric liquid of the inventioncomprises at least one alkylaryl hydrocarbon having at least threenon-condensed aromatic rings.

[0050] Even more preferably, the dielectric liquid of the inventioncomprises at least one alkylaryl hydrocarbon having at least threenon-condensed aromatic rings in a quantity of not less than 10 wt % onthe total weight of the dielectric liquid.

[0051] Preferably, the dielectric liquid of the invention comprises atleast one alkylaryl hydrocarbon having the structural formula:

[0052] wherein:

[0053] R1, R2, R3 and R4, equal or different, are hydrogen or methyl;

[0054] n1 and n2, equal or different, are zero, 1 or 2, with the provisothat the sum n1+n2 is less than or equal to 3.

[0055] The dielectric liquid can also contain minor quantities of atleast one triphenylmethane, either unsubstituted or substituted by atleast one radical selected from methyl, benzyl and methylbenzyl.Examples of triphenylmethanes are: ditoluylphenylmethane,dixylylphenylmethane, xylyltoluylphenylmethane and the like, or theirmixtures.

[0056] More preferably, the dielectric liquid of the invention comprisesat least one alkylaryl hydrocarbon of the aforegiven formula (I) inwhich the sum n1+n2 is other than zero.

[0057] Alkylaryl hydrocarbons corresponding to formula (I) in which thesum n1+n2 is equal to zero, and usable advantageously in this invention,are for example: benzyltoluene, benzylxylene, (methylbenzyl)toluene,(methylbenzyl)xylene and the like, or their mixtures.

[0058] Alkylaryl hydrocarbons corresponding to formula (I) in which thesum n1+n2 is other than zero, and usable advantageously in thisinvention, are for example: dibenzyltoluene, dibenzylxylene,di(methylbenzyl)toluene, di(methylbenzyl)xylene and the like, or theirmixtures.

[0059] The alkylaryl hydrocarbons of formula (I) are generally preparedby reacting benzylchloride, methylbenzylchloride or their mixtures, withan aromatic hydrocarbon selected from benzene, toluene, xylene or theirmixtures, in the presence of a Friedel-Crafts catalyst (for exampleFeCl₃, SbCl₃, TiCl₄ or AlCl₃). Further details regarding the preparationof alkylaryl hydrocarbons of formula (I) are given for example in U.S.Pat. Nos. 5,192,463, 5,446,228, 5,545,355 and 5,601,755.

[0060] The dielectric liquid suitable for implementing the invention hasgood heat resistance, considerable gas absorption capacity, inparticular for hydrogen, and hence high resistance to partialdischarges, so that dielectric loss is not high even at high temperatureand high electrical gradient. The weight ratio of dielectric liquid tobase polymer material of the invention is generally between 1:99 and25:75, preferably between 2:98 and 20:80, and more preferably between3:97 and 15:85.

[0061] According to a preferred aspect, the cable of the invention hasat least one extruded covering layer with electrical insulationproperties formed from the thermoplastic polymer material in admixturewith the aforedescribed dielectric liquid.

[0062] According to a further preferred embodiment, the cable of theinvention has at least one extruded covering layer with semiconductiveproperties formed from the thermoplastic polymer material in admixturewith the aforedescribed dielectric liquid. To form a semiconductivelayer, a conductive filler is generally added to the polymer material.To ensure good dispersion of the conductive filler within the basepolymer material, this latter is preferably selected from propylenehomopolymers or copolymers comprising at least 40 wt % of amorphousphase, on the total polymer weight.

[0063] In a preferred embodiment, the cable of the invention has atleast one electrical insulation layer and at least one semiconductivelayer formed from a thermoplastic polymer material in admixture with adielectric liquid as hereinabove described. This prevents thesemiconductive layers from absorbing, with time, part of the dielectricliquid present in the insulating layer, so reducing its quantity just atthe interface between the insulating layer and semiconductive layer, inparticular the inner semiconductive layer where the electrical field ishigher.

[0064] According to a further aspect, the invention relates to a polymercomposition comprising a thermoplastic polymer material in admixturewith a dielectric liquid, in which:

[0065] said thermoplastic material comprises a propylene homopolymer ora copolymer of propylene with at least one olefin comonomer selectedfrom ethylene and an α-olefin other than propylene, said homopolymer orcopolymer having a melting point of greater than or equal to 140° C. anda melting enthalpy of from 30 to 100 J/g;

[0066] said liquid comprises at least one alkylaryl hydrocarbon with atleast two non-condensed aromatic rings and a ratio of number of arylcarbon atoms to total number of carbon atoms greater than or equal to0.6, preferably greater than or equal to 0.7.

[0067] According to a further aspect, the invention relates to the useof a polymer composition, as described hereinabove, as the base polymermaterial for preparing a covering layer (4) with electrical insulationproperties, or for preparing a covering layer (3, 5) with semiconductiveproperties.

[0068] In forming a covering layer for the cable of the invention, otherconventional components can be added to the aforedefined polymercomposition, such as antioxidants, processing aids, water treeretardants, and the like.

[0069] Conventional antioxidants suitable for the purpose are forexample distearyl-thiopropionate and pentaerithryl-tetrakis[3-(3,5-di-tertbutyl-4-hydroxyphenyl)propionate] and the like, or theirmixtures.

[0070] Processing aids which can be added to the polymer base include,for example, calcium stearate, zinc stearate, stearic acid, paraffin waxand the like, or mixtures thereof.

[0071] With particular reference to medium and high voltage cables, thepolymer materials as defined hereinabove can be advantageously used toform an insulating layer. As stated above, these polymer materials showindeed good mechanical characteristics both at ambient temperature andunder hot conditions, and also show improved electrical properties. Inparticular they enable high operating temperature to be employed,comparable with or even exceeding that of cables with coveringsconsisting of crosslinked polymer base materials.

[0072] If a semiconductive layer is to be formed, a conductive filler,in particular carbon black, is generally dispersed within the polymermaterial in a quantity such as to provide the material withsemiconductive characteristics (i.e. such as to obtain a resistivity ofless than 5 Ohm.m at ambient temperature). This quantity is generallybetween 5 and 80 wt %, and preferably between 10 and 50 wt %, of thetotal weight of the mixture.

[0073] The possibility to use the same type of polymer composition forboth the insulating layer and the semiconductive layers is particularlyadvantageous in producing cables for medium or high voltage, in that itensures excellent adhesion between adjacent layers and hence betterelectrical behaviour, particularly at the interface between theinsulating layer and the inner semiconductive layer, where theelectrical field and hence the risk of partial discharges are higher.

[0074] The compositions of the invention can be prepared by mixingtogether the base polymer material, the dielectric liquid and any otheradditives possibly present by methods known in the art. Mixing can becarried out for example by an internal mixer of the type with tangentialrotors (Banbury) or with interpenetrating rotors, or, preferably, in acontinuous mixer of Ko-Kneader (Buss) type, or of co- orcounter-rotating double-screw type.

[0075] Alternatively, the dielectric liquid of the invention can beadded to the polymer material during the extrusion step by directinjection into the extruder cylinder.

[0076] According to the present invention, the use of the aforedefinedpolymer composition in covering cables for medium or high voltageenables recyclable, flexible coverings to be obtained with excellentmechanical and electrical properties.

[0077] Greater compatibility has also been found between the dielectricliquid and thermoplastic base polymer of the invention than in the caseof similar mixtures of the same polymer material with other dielectricliquids known in the art. This greater compatibility leads, inter alia,to less exudation of the dielectric liquid and hence a reduction of thealready discussed migration phenomena. Because of their high operatingtemperature and their low dielectric loss, the cables of the inventioncan carry, for the same voltage, a power at least equal to or evengreater than that transportable by a traditional cable with XLPEcovering.

[0078] For the purposes of the invention the term “medium voltage”generally means a voltage of between 1 and 35 kV, whereas “high voltage”means voltages higher than 35 kV.

[0079] Although this description is mainly focused on the production ofcables for transporting or distributing medium or high voltage energy,the polymer composition of the invention can be used for coveringelectrical devices in general and in particular cables of differenttype, for example low voltage cables, telecommunications cables orcombined energy/telecommunications cables, or accessories used inconstructing electrical lines, such as terminals or connectors.

[0080] Further characteristics will be apparent from the detaileddescription given hereinafter with reference to the accompanyingdrawing, in which:

[0081]FIG. 1 is a perspective view of an electric cable, particularlysuitable for medium or high voltage, according to the invention.

[0082] In FIG. 1, the cable 1 comprises a conductor 2, an inner layerwith semiconductive properties 3, an intermediate layer with insulatingproperties 4, an outer layer with semiconductive properties 5, a metalscreen 6, and an outer sheath 7.

[0083] The conductor 2 generally consists of metal wires, preferably ofcopper or aluminium, stranded together by conventional methods. At leastone covering layer selected from the insulating layer 4 and thesemiconductive layers 3 and 5 comprises the composition of the inventionas heretofore defined. Around the outer semiconductive layer 5 there isusually positioned a screen 6, generally of electrically conductingwires or strips wound helically. This screen is then covered by a sheath7 of a thermoplastic material, for example non-crosslinked polyethylene(PE) or preferably a propylene homopolymer or copolymer as heretoforedefined.

[0084] The cable can also be provided with an outer protective structure(not shown in FIG. 1) the main purpose of which is to mechanicallyprotect the cable against impact or compression. This protectivestructure can be, for example, a metal reinforcement or a layer ofexpanded polymer as described in WO 98/52197.

[0085]FIG. 1 shows only one possible embodiment of a cable according tothe invention. Suitable modifications known in the art can evidently bemade to this embodiment, but without departing from the scope of theinvention.

[0086] The cable of the invention can be constructed in accordance withknown methods for depositing layers of thermoplastic material, forexample by extrusion. The extrusion is advantageously carried out in asingle pass, for example by the tandem method in which individualextruders are arranged in series, or by co-extrusion with a multipleextrusion head.

[0087] The following examples illustrate the invention, but withoutlimiting it.

EXAMPLES

[0088] Table 1 shows the characteristics of the dielectric liquids usedin the following examples. TABLE 1 Dielectric Dielectric Total carbonRatio liquid constant (*) atoms C(aryl)/C(total) Jarylec^(R) 2.8 MXX =16 0.75 Exp 4 DXX = 24 Jarylec^(R) 2.7 21 0.86 Exp 3 Baysilone^(R) 2.6 ——

[0089] The dielectric liquids according to the invention were:

[0090] Jarylec^(R)Exp4 (commercial product of Elf Atochem):

[0091] a mixture containing 85 wt % of monoxylylxylene (MXX)

[0092] and 15 wt % of dixylylxylene (DXX)

[0093] Jarylec^(R)Exp3 (commercial product of Elf Atochem):

[0094] dibenzyltoluene (DBT)

[0095] The comparison dielectric liquids were:

[0096] Baysilone^(R)PD5 (commercial product of General Electric—Bayer):

[0097] polyphenylmethylsiloxane (PPMS), polyaromatic dielectric oil asdescribed in IEEE Transactions on Electrical Insulation Vol. 26, No.4,1991, having a viscosity of 4 mm²/sec at 25° C.;

[0098] Flexon^(R)641 (commercial product of Esso):

[0099] naphthene-based aromatic oil having a viscosity of 22 mm²/sec at40° C., consisting of 40 wt % aromatic hydrocarbons, 57 wt % saturatedhydrocarbons and 3 wt % polar compounds.

[0100] The following polymer materials were used:

[0101] a flexible propylene homopolymer with melting point 160° C.,melting enthalpy 56.7 J/g, MFI 1.8 dg/min and flexural modulus 290 MPa(Rexflex^(R)WL105—commercial product of Huntsman Polymer Corp.)(Examples 1-6)

[0102] a propylene heterophase copolymer with an ethylene/propyleneelastomeric phase content of about 65 wt % (propylene 72 wt % in theelastomeric phase), melting enthalpy 32 J/g, melting point 163° C., MFI0.8 dg/min and flexural modulus of about 70 MPa(Hifax^(R)KSO81—commercial product of Montell).

[0103] Composition Preparation

[0104] The polymer in granular form was preheated to 80° C. in aturbomixer. The dielectric liquid was added, in the quantities specifiedfor the formulations given in Table 2, to the polymer preheated in theturbomixer under agitation at 80° C. over 15 min. After the additionagitation was continued for a further hour at 80° C. until the liquidwas completely absorbed in the polymer granules.

[0105] After this first stage, the resultant material was kneaded in alaboratory double-screw Brabender Plasticorder PL2000 at a temperatureof 185° C. to complete homogenization. The material left thedouble-screw mixer in the form of granules.

[0106] Measurement of Dielectric Strength (DS)

[0107] The dielectric strength of the polymer compositions obtained wasevaluated on test-pieces of insulating material having the geometryproposed by the EFI (Norwegian Electric Power Research Institute) in thepublication “The EFI Test Method for Accelerated Growth of Water Trees”(IEEE International Symposium on Electrical insulation, Toronto, Canada,Jun. 3-6, 1990). In this method, the cable is simulated withglass-shaped test pieces of insulating material having their base coatedon both sides with a semiconductive material coating.

[0108] The glass-shaped test-pieces were formed by moulding discs ofinsulating material at 160-170° C. from a plate of thickness 10 mmobtained by compressing granules at about 190° C.

[0109] The inner and outer surfaces of the base, which had a thicknessof about 0.40-0.45 mm, were coated with a semiconductive coating. The DSmeasurement was made by applying to these specimens, immersed insilicone oil at 20° C., an alternating current at 50 Hz starting with avoltage of 25 kV and increasing in steps of 5 kV every 30 minutes untilperforation of the test-piece occurred. Each measurement was repeated on10 test-pieces. The values given in Table 2 are the arithmetic mean ofthe individual measured values. TABLE 2 Dielectric % dielectric DS Ex.Polymer liquid liquid by weight (mean) 1* Rexflex^(R) — —  92 WL 105 2*Rexflex^(R) Baysilone^(R) 5  90 WL 105 PD5 3* Rexflex^(R) Flexon^(R)6415  94 WL 105 4  Rexflex^(R) Jarylec^(R) 6 128 WL 105 Exp4 5  Rexflex^(R)Jarylec^(R) 15 150 WL 105  EXP4 6  Rexflex^(R) Jarylec^(R) 4 143 WL 105Exp3 7* Hifax^(R) — —  90 KS081 8  Hifax^(R) Jarylec^(R) 15 140 KS081Exp4

[0110] The dielectric strength values given in Table 2 highlight theimprovement in electrical performance deriving from the dielectricliquids of the invention, compared to that of the base polymer as suchor when mixed with the comparison dielectric liquids.

[0111] Migration Tests

[0112] Using the polymer/dielectric liquid compositions prepared inExamples 5 and 6 moulded into 5 mm plates at 190° C., the loss ofdielectric liquid (expressed as percentage by weight on the initialquantity) was measured against time at 20° C. in air in order to verifythe diffusivity of the dielectric liquids in the polymer and hence theirstability with time in these compositions. TABLE 3 CompositionComposition Days Example 6 Example 3 0 100.00 100.00 1 100.00 99.84 499.97 99.32 5 99.97 99.14 6 99.97 99.14 8 99.75 98.6 12  99.45 97.91 18 99.34 96.69 28  99.24 94.92 39  99.14 93.54

[0113] The data of FIG. 3 show the high compatibility of the dielectricliquids with the described base polymer material and consequently thelow tendency of these liquids to migrate to the outside of the polymermaterial.

1. A cable (1) comprising at least one electrical conductor (2) and atleast one extruded covering layer (3, 4, 5) based on a thermoplasticpolymer material in admixture with a dielectric liquid, wherein: saidthermoplastic material comprises a propylene homopolymer or a copolymerof propylene with at least an olefin comonomer selected from ethyleneand an α-olefin other than propylene, said homopolymer or copolymerhaving a melting point greater than or equal to 140° C. and a meltingenthalpy of from 30 to 100 J/g; said liquid comprises at least onealkylaryl hydrocarbon with at least two non-condensed aromatic rings anda ratio of number of aryl carbon atoms to total number of carbon atomsgreater than or equal to 0.6.
 2. A cable as claimed in claim 1, whereinthe ratio of number of aryl carbon atoms to total number of carbon atomsis greater than or equal to 0.7.
 3. A cable as claimed in claim 1 or 2,wherein the propylene homopolymer or copolymer has a melting point offrom 145 to 170° C.
 4. A cable as claimed in any one of the precedingclaims, wherein the propylene homopolymer or copolymer has a meltingenthalpy of from 30 to 85 J/g.
 5. A cable as claimed in any one of thepreceding claims, wherein the propylene homopolymer or copolymer has aflexural modulus, measured at ambient temperature, of from 30 to 1400MPa.
 6. A cable as claimed in any one of the preceding claims, whereinthe propylene homopolymer or copolymer has a flexural modulus, measuredat ambient temperature, of from 60 to 1000 MPa.
 7. A cable as claimed inany one of the preceding claims, wherein the propylene homopolymer orcopolymer has a melt flow index, measured at 230° C., of from 0.05 to10.0 dg/min.
 8. A cable as claimed in any one of the preceding claims,wherein the propylene homopolymer or copolymer has a melt flow index,measured at 230° C., of from 0.5 to 5.0 dg/min.
 9. A cable as claimed inany one of the preceding claims, wherein the olefin comonomer is presentin a quantity of less than or equal to 15 mol %.
 10. A cable as claimedin any one of the preceding claims, wherein the olefin comonomer ispresent in a quantity of less than or equal to 10 mol %.
 11. A cable asclaimed in any one of the preceding claims, wherein the olefin comonomeris ethylene or an α-olefin of formula CH₂═CH—R, where R is a linear orbranched C₂—C₁₀ alkyl.
 12. A cable as claimed in the preceding claim,wherein the α-olefin is selected from 1-butene, 1-pentene,4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene and thelike, or combinations thereof.
 13. A cable as claimed in any one of thepreceding claims, wherein the thermoplastic material is selected from:a) a propylene homopolymer or a copolymer of propylene with at least oneolefin comonomer selected from ethylene and an α-olefin other thanpropylene, having a flexural modulus of from 30 to 900 MPa; b) aheterophase copolymer comprising a thermoplastic phase based onpropylene and an elastomeric phase based on ethylene copolymerized withan α-olefin, in which the elastomeric phase is present in a quantity ofat least 45 wt % on the total weight of the heterophase copolymer.
 14. Acable as claimed in the preceding claim, wherein the propylenehomopolymer or copolymer under a) has a flexural modulus of from 50 to400 MPa.
 15. A cable as claimed in claim 13 or 14, wherein the propylenehomopolymer or copolymer under a) has: a melting point of from 140 to165° C.; a melting enthalpy of from 30 to 80 J/g; a fraction soluble inboiling diethyl ether in an amount of less than or equal to 12 wt %,having a melting enthalpy of less than or equal to 4 J/g; a fractionsoluble in boiling n-heptane in an amount of from 15 to 60 wt %, havinga melting enthalpy from 10 to 40 J/g; and a fraction insoluble inboiling n-heptane in an amount of from 40 to 85 wt %, having a meltingenthalpy of greater than or equal to 45 J/g.
 16. A cable as claimed inany one of claims from 13 to 15, wherein the propylene homopolymer orcopolymer of a) has: a fraction soluble in boiling diethyl ether in anamount of from 1 to 10 wt %, having a melting enthalpy of less than orequal to 2 J/g; a fraction soluble in boiling n-heptane in an amount offrom 20 to 50 wt %, having a melting enthalpy of from 15 to 30 J/g; anda fraction insoluble in boiling n-heptane in an amount of from 50 to 80wt %, having a melting enthalpy from 50 to 95 J/g.
 17. A cable asclaimed in claim 13, wherein the α-olefin included in the elastomericphase of the heterophase copolymer under b) is propylene.
 18. A cable asclaimed in the preceding claim, wherein the elastomeric phase consistsof an elastomeric copolymer of ethylene and propylene comprising from 15to 50 wt % of ethylene and from 50 to 85 wt % of propylene on the weightof the elastomeric phase.
 19. A cable as claimed in any one of thepreceding claims, wherein the base thermoplastic material is thepropylene homopolymer or copolymer in mechanical mixture with a lowcrystallinity polymer having a melting enthalpy of less than or equal to30 J/g, and a quantity of less than or equal to 70 wt % on the totalweight of the thermoplastic material.
 20. A cable as claimed in thepreceding claim, wherein the low crystallinity polymer is in a quantityof from 20 to 60 wt % on the total weight of the thermoplastic material.21. A cable as claimed in claims 19 or 20, wherein the low crystallinitypolymer is a copolymer of ethylene with a C₃—C₁₂ α-olefin.
 22. A cableas claimed in claim 19 or 20, wherein the low crystallinity polymer is acopolymer of ethylene with an α-olefin and a diene.
 23. A cable asclaimed in claims 21 or 22, wherein the ethylene copolymer is selectedfrom i) a copolymer having the following monomer composition: 35-90 mol% of ethylene; 10-65 mol % of α-olefin; 0-10 mol % of a diene; ii) acopolymer having the following monomer composition: 75-97 mol % ofethylene; 3-25 mol % of α-olefin; 0-5 mol % of a diene.
 24. A cable asclaimed in the preceding claim, wherein the ethylene copolymer isselected from a copolymer having the following monomer composition:90-95 mol % of ethylene; 5-10 mol % of α-olefin; 0-2 mol % of a diene.25. A cable as claimed in any one of claims from 21 to 24, wherein theα-olefin is selected from propylene, 1-hexene and 1-octene.
 26. A cableas claimed in any one of claims from 22 to 25, wherein the diene hasfrom 4 to 20 carbon atoms.
 27. A cable as claimed in any one of claimsfrom 22 to 26, wherein the diene is selected from a conjugated ornon-conjugated linear diolefin, and a monocyclic or polycyclic diene.28. A cable as claimed in any one of claims from 22 to 27, wherein thediene is selected from 1,3-butadiene, 1,4-hexadiene, 1,6-octadiene,1,4-cyclohexadiene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene,5-vinyl-2-norbornene, or their mixtures and the like.
 29. A cable asclaimed in any one of the preceding claims, wherein the alkylarylhydrocarbon has a dielectric constant at 25° C. of less than or equal to3.5.
 30. A cable as claimed in any one of the preceding claims, whereinthe dielectric constant at 25° C. is less than or equal to
 3. 31. Acable as claimed in any one of the preceding claims, wherein thedielectric liquid has a kinematic viscosity at 20° C. of between 1 and500 mm²/s.
 32. A cable as claimed in any one of the preceding claims,wherein the dielectric liquid has a kinematic viscosity at 20° C. ofbetween 5 and 100 mm²/s.
 33. A cable as claimed in any one of thepreceding claims, wherein the alkylaryl hydrocarbon has a hydrogenabsorption capacity of greater than or equal to 5 mm³/min.
 34. A cableas claimed in the preceding claim, wherein the hydrogen absorptioncapacity is greater than or equal to 50 mm³/min.
 35. A cable as claimedin any one of the preceding claims, wherein an epoxy resin is added tothe dielectric liquid in a quantity of less than or equal to 1 wt % onthe weight of the liquid.
 36. A cable as claimed in the preceding claim,wherein the alkylaryl hydrocarbon is present in a quantity of greaterthan or equal to 10 wt % on the total weight of the dielectric liquid.37. A cable as claimed in any one of the preceding claims, wherein thedielectric liquid comprises at least one alkylaryl hydrocarbon having atleast three non-condensed aromatic rings.
 38. A cable as claimed in anyone of the preceding claims, wherein the alkylaryl hydrocarbon has thestructural formula:

where: R1, R2, R3 and R4, equal or different, are hydrogen or methyl; n1and n2, equal or different, are zero, 1 or 2, with the proviso that thesum n1+n2 is less than or equal to
 3. 39. A cable as claimed in any oneof the preceding claims, wherein the alkylaryl hydrocarbon is selectedfrom benzyltoluene, benzylxylene, (methylbenzyl)toluene,(methylbenzyl)xylene, dibenzyltoluene, dibenzylxylene,di(methylbenzyl)toluene, di(methylbenzyl)xylene and the like, or theirmixtures.
 40. A cable as claimed in any one of the preceding claims,wherein the dielectric liquid comprises at least one triphenylmethane,either unsubstituted or substituted by at least one radical selectedfrom methyl, benzyl and methylbenzyl.
 41. A cable as claimed in thepreceding claim, wherein the triphenylmethane is selected fromditoluylphenylmethane, dixylylphenylmethane, xylyltoluylphenylmethaneand the like, or their mixtures.
 42. A cable as claimed in any one ofthe preceding claims, wherein the weight ratio of dielectric liquid tobase polymer material is from 1:99 to 25:75.
 43. A cable as claimed inany one of the preceding claims, wherein the weight ratio of dielectricliquid to base polymer material is from 2:98 to 20:80.
 44. A cable asclaimed in any one of the preceding claims, wherein the weight ratio ofdielectric liquid to base polymer material is from 3:97 to 15:85.
 45. Acable as claimed in any one of the preceding claims, wherein theextruded covering layer is a layer (4) with electrical insulationproperties.
 46. A cable as claimed in any one of claims from 1 to 45,wherein the extruded covering layer is a layer (3, 5) withsemiconductive properties.
 47. A cable as claimed in the precedingclaim, wherein a conductive filler is dispersed in the layer withsemiconductive properties.
 48. A cable as claimed in any one of thepreceding claims, wherein the base polymer material is selected frompropylene homopolymers or copolymers comprising at least 40 wt % ofamorphous phase, on the total polymer weight.
 49. A cable as claimed inany one of the preceding claims, wherein at least one layer withelectrical insulation properties and at least one layer withsemiconductive properties are present.
 50. A polymer compositioncomprising a thermoplastic polymer material in admixture with adielectric liquid in accordance with any one of claims from 1 to
 49. 51.Use of a polymer composition as claimed in claim 50, as base polymermaterial for the preparation of a covering layer (4) with electricalinsulation properties.
 52. Use of a polymer composition as claimed inclaim 50, as base polymer material for the preparation of a coveringlayer (3, 5) with semiconductive properties.